The recognition of MHC molecules loaded with pathogen-derived
peptides by T cells initiates the immune response. My laboratory is
interested in how human class II molecules acquire their antgenic peptide
cargo. Specifically, we are interested in the role that 2 accessory
molecules, HLA-DM and HLA-DO, play in the class II antigen-processing
pathway. DM facilitates peptide loading of class II molecules, while DO
specifically inhibits the the peptide loading ability of DM.

We believe that the interplay between these 2 molecules has a fundamental
role in the modulation of MHC class II-mediated immune responses. The
powerful and destructive nature of an immune response must be regulated in
a way such that healthy cells are not destroyed during the process of
fighting off disease. When the immune system is not correctly regulated,
autoimmune disease can occur. The modulation of antigen presentation by DM
and DO clearly present a method of controlling the immune response and
preventing autoimmune disease.

The Class II Antigen-Processing Pathway

Over the past decade, the basic mechanisms involved in the generation of
major histocompatibility complex (MHC) class II-peptide complexes have been
worked out. The class II alpha and beta subunits assemble in the ER with
another transmembrane glycoprotein called the invariant (I) chain. The fully
assembled nonameric [(alpha-beta-I)3] complex exits the ER and moves through
the Golgi stacks, where it is sorted by a signal in the cytoplasmic domain
of the I chain into late endosomes and lysosomes. In these acidic
compartments, the I chain is proteolytically cleaved from the class II
molecules by resident proteases until only a small peptide fragment of the I
chain (class II-associated I-chain peptides or CLIP) remains bound in the
peptide-binding groove of the class II molecules.

Next, another class II-like molecule, HLA-DM (H2-M in mice), associates
with the class II-CLIP complex, forcing CLIP from the binding groove of the
class II molecules. DM remains bound to the "peptide-free" class II
dimers, stabilizing them until appropriate antigenic peptides derived from
engulfed extracellular antigens (such as bacteria) are generated and loaded
onto the empty class II molecules. DM also "edits" the peptide repertoire,
ensuring that only high-affinity peptides are presented on the cell surface.

The resulting class II-peptide complexes are released from DM and
transported, by a poorly defined process, to the cell surface where they are
presented to T cells. CD4 T cell recognition of such a complex results in an
immune response to the engulfed antigen.

HLA-DO Inhibits DM-mediated Class II Peptide Loading

HLA-DO (DO) (H-2O in mice) is another class II-like molecule that is
expressed in B cells and thymic epithelia but not in other APC (i.e.,
macrophages and dendritic cells). Association of the DO ab heterodimer with
DM in the ER is required for DO transport to endosomal compartments, where
DO/DM complexes accumulate. This suggests that DO plays a unique role in the
class II processing pathway, specifically in B cells, either by regulating
or altering DM function.

Our initial biochemical and cell transfection analyses clearly showed that
DO inhibited DM-mediated peptide loading, resulting in a down modulation of
the class II processing pathway. However, recent studies have
suggested that the function of DO may be more multifaceted. Depending on
both the experimental system and the Ag studied, it has been shown that DO
can inhibit, promote, or have no effect on class II peptide loading.

Collectively, these studies suggest that DO modifies the loading of specific
peptides on to class II molecules, which results in a change in the class II
peptide repertoire. However, the biological relevance of a cell-specific
modulator of DM function has proven to be enigmatic, and it remains to be
determined if modulation of DM activity is the only or even the primary
function of DO. Thus, the main focus of our lab is to determine the function
DO/H2-O and to determine the functional consequence of DO/H2-O expression in
vivo during the course of immune responses.